The present invention relates to an assembly for connection of a flexible tubular pipe to an undersea installation in order to transport liquid and/or gaseous hydrocarbons, for example liquefied natural gas or water.
The extraction and transport of these fluids in a marine environment requires an undersea installation at the bottom of the sea, and an overlying surface installation, such as to be able to recuperate the hydrocarbons by means of a flexible tubular pipe which extends between the undersea installation and the surface installation.
This flexible pipe is advantageously of the unbonded type, and is described in the standardizing documents published by the American Petroleum Institute API 17 J and API RP 17B. Without departing from the scope of the invention, the flexible pipe could be a flexible pipe of the bonded type, or a power or control umbilical unit for example. Also, the invention is not limited to a flexible pipe, but also extends to a bundle of flexible pipes. The undersea installation can for example be a well head or a connection of the FLET or PLET type.
The surface installation is subjected to movements of swell, currents and wind, thus involving displacement of the surface installation both in the horizontal direction and in the vertical direction. These movements of the surface installation added to the currents are transmitted to the flexible tubular pipe and are propagated along the pipe, consequently generating movements and deformations of the flexible tubular pipe. The pipe comprises a floating portion between the surface and the bottom, and a contact portion, the latter comprising a zone of contact of the flexible tubular pipe with the bottom of the sea (touch down zone or TDZ), with the position of the contact zone TDZ being displaced according to these movements, and in particular those of the surface installation. At the level of the contact zone, the contact portion has curvature, and then continues its course along the bottom of the sea towards the undersea installation. The movements which are transmitted to the flexible tubular pipe can be propagated as far as the contact zone or TDZ and generate significant deformations of the flexible pipe, such that the torsion or flexure of the pipe can result in compression of the flexible pipe at the bottom, or even crush the pipe. Also, these movements and deformations of the pipe exert significant forces on the pipe in contact with the bottom of the sea, particularly at the level of the contact zone (TDZ), which can lead to deterioration of the intactness of the flexible pipe.
In order to limit the forces which are exerted on the pipe at the level of the contact zone, different configurations of flexible pipe exist, to uncouple the movements of the surface installation from those of the pipe at the level of the TDZ.
In situations in which the sea is shallow, or there is a shoal, typically where the level is less than 100 meters relative to the bottom of the sea, the flexible tubular pipe configurations generally have long excess lengths in order to accommodate the movements which are transmitted to the flexible pipe. According to the movements of the surface installation and the currents, the flexible pipe is deformed and displaced so as to absorb these forces.
A typical configuration for shallow seas is that in which the flexible tubular pipe, which has two opposite ends, one connected to the undersea installation and the other connected to the surface installation, is equipped between the two with undersea buoys and heavy modules which form ballasts. The undersea buoys make it possible to form a floating portion of flexible tubular pipe, which is thus maintained between the bottom of the sea and the surface, and describes two curves with concavity which faces towards the bottom, whereas, in particular in the vicinity of the undersea installation, the heavy modules make it possible to maintain a contact portion of the tubular pipe in contact with the bottom of the sea. This configuration is commonly known as the double wave configuration.
Reference can be made in particular to document US 2006/0159 521, which shows an installation in which the flexible tubular pipe is equipped with undersea protection at the level of the zones of contact with the bottom of the sea.
Thus, under the effect of the movements of the surface installation and the currents, the floating portion of flexible tubular pipe, which is retained by the surface end connected to the surface installation, is moved and deformed so as to accommodate these movements and to dissipate at least partially the forces which are transmitted at the level of the contact zones. When the meteorological conditions deteriorate and the surface installation is moved relative to the undersea installation with high amplitudes, typically of approximately the height of a wave for example, and at high frequencies, in its so-called double wave configuration the flexible pipe is no longer sufficient to avoid the strong stresses on the pipe at the level of its zones of contact with the bottom of the sea.
Consequently, a problem which arises, and which the present invention aims to solve, consists of providing an assembly for connection of a flexible pipe to an undersea installation in a shallow sea, which makes it possible to protect the undersea installation when the meteorological conditions have deteriorated and the sea is rough.
Another problem which the invention aims to solve is that of providing an assembly for connection of a flexible pipe to an undersea installation, which makes it possible to protect the flexible tubular pipe at the level of the contact zone, by dissipating the forces transmitted at the level of the contact zone (TDZ).
Yet another objective of the invention is to control the excessive displacements of the pipe when the sea is rough.